Motor vehicle transmissions of the type addressed by this invention include several fluid operated torque transmitting devices, referred to herein as clutches, which are automatically engaged and disengaged according to a predefined pattern to establish different speed ratios between input and output shafts of the transmission. The input shaft is coupled to an internal combustion engine through a fluid coupling such as a torque converter, and the output shaft is mechanically connected to drive one or more vehicle wheels.
The various speed ratios of the transmission are typically defined in terms of the ratio Ni/No, where Ni is the input shaft speed and No is the output shaft speed. Speed ratios having a relatively high numerical value provide a relatively low output speed and are generally referred to as lower speed ratios; speed ratios having a relatively low numerical value provide a relatively high output speed and are generally referred to as upper speed ratios.
Shifting between the various speed ratios generally involves disengaging a clutch associated with the current or actual speed ratio Ract by controlling clutch fluid pressure with a first fluid valve, and engaging a clutch associated with the desired speed ratio Rdes by controlling clutch fluid pressure with a second fluid valve. The clutch to be released is referred to as the off-going clutch, while the clutch to be engaged is referred to as the on-coming clutch. Shifts of this type are referred to as clutch-to-clutch in that no speed responsive or freewheeling elements are used.
Conventional clutch-to-clutch controls are mechanized such that each of such clutches has associated therewith a dedicated fluid valve. Mechanizations are known utilizing a fluid operated clutch priority valve responsive to fluid pressures of the various fluid valves to prevent transmission tie ups caused by clutches having undesirable simultaneous capacity. The present invention is directed toward an override control of a clutch priority valve in a clutch-to-clutch control having a single fluid valve controlling two gear ratio clutches and a clutch priority valve mechanized such that the fluid pressures associated with the two gear ratio clutches controlled by the single fluid valve are indistinguishable thereby.